Not sure what devices are available, but you might seek out an orthodics specialist who can work with your dad. Before going the device route, you might try stretching exercises. Stretching can often alleviate problems with muscle tightening. Stretching has helped me a lot with my legs and feet issues.

There is also a device called Yoga Toes which helps straighten the toes and relax the muscles. You dont put them in your shoes, but rather wear it around the house when you are shoeless.....Ive tried them and although they are not the most comfortable thing to walk in, its great to stretch and relax your toes while watching TV or sitting at my desk. My Neuro told me about them when I was having some walking problems, he noticed that I too, was curling my toes when I walked. Check it out, you can find them most anywhere that sells Pedicure items and Ortho supports.
Nikki